Parameter estimation for adaptive antenna system

ABSTRACT

There is proposed a technique for determining parameters, especially channel estimation, in the receiver of a communication system utilizing adaptive antenna techniques. The technique uses information transmitted in the antenna signals and in the beam signals to determine the parameters.

This application is a continuation of U.S. patent application Ser. No.10/465,995, filed Nov. 24, 2003, which is a 371 Application ofInternational Application No. PCT/EP01/15152, filed Dec. 20, 2001.

FIELD OF THE INVENTION

The present invention relates to a technique for estimating parametersin the receiver of an adaptive antenna system, and particularly but notexclusively to channel estimation in the receiver of a mobile station ina mobile communication system.

BACKGROUND TO THE INVENTION

In wide-band code division multiple access (W-CDMA) systems, one of themost demanding tasks having a significant effect on the performance ofthe receiver is the channel estimation, in which the complex channelcoefficients are estimated. The goal of channel estimation is to cancelthe distortion caused to the transmitted signal by the radio channelsuch that as perfect a replica of the transmitted signal is retrieved asis possible.

This can be achieved by coherent demodulation in which the receivedsignal is multiplied with the complex conjugated channel estimate.However, in multi-path fast fading conditions the coherent detection isdifficult to achieve and a channel estimation method is required thatworks satisfactorily under the challenging fast fading circumstances.

The channel multi-path profile heavily affects the performance of thechannel estimation due to the fact that the estimation must be doneindividually in each temporal rake finger of the rake receiver. The moremulti-path components in the channel, the lower the signal-to-noiseratio (SNR) per rake finger, since the channel energy is distributed inthe propagation paths.

In the forward link, i.e. the down-link, of a wide-band code divisionmultiple access (WCDMA) system, a primary common pilot channel (P-CPICH)is broadcast over the entire cell or a sector. The P-CPICH is broadcastalso in the case of a multi-beam arrangement (multiple beams per sector)and in user specific beam-forming. Therefore there always exists onesuch channel per sector regardless of the applied transmission scheme.

In systems utilizing adaptive antenna techniques, dedicated channels areusually transmitted through a narrow beam, which means that the P-CPICHand the down-link dedicated physical channels (DL-DPCH) usuallyexperience different channel characteristics on transmission to a mobilestation antenna.

In current known systems, the down-link dedicated physical controlchannel (DL-DPCCH) is used as the phase reference for the DL-DPCH inadaptive antenna systems because of the fact that the P-CPICH does notexperience the same channel characteristics.

It is therefore an aim of the present invention to provide an improvedtechnique for estimating parameters in the receiver of an adaptiveantenna system.

SUMMARY OF THE INVENTION

In one aspect of the present invention, a disclosed method includesreceiving a beam signal at a receiver of a communication system, thereceiver having an adaptive antenna transmitter. The communicationsystem is a cellular system including a plurality of cells, each cellincluding at least one sector. An antenna signal is received at thereceiver. Furthermore, parameters of the received beam signal areestimated based on information received in the received antenna signal.Specifically, the antenna signal is transmitted over an entire sectorand the beam signal is transmitted over part of a sector and theestimating of the parameters is further based on the received beamsignal.

In one aspect of the present invention, a disclosed method includesreceiving a beam signal having a dedicated channel at a receiver, thereceiver having an adaptive antenna transmitter. The communicationsystem is a cellular system including a plurality of cells, each cellincluding at least one sector. An antenna signal is received at thereceiver and the antenna signal has a primary common pilot channel.Channel estimation is performed on the received beam signal based onpilot signals received in the primary common pilot channel. The antennasignal is transmitted over an entire sector and the beam signal istransmitted over part of a sector and the channel estimation isadditionally based on dedicated signals received in the dedicatedphysical channel.

In another aspect of the present invention, a disclosed apparatusincludes an adaptive antenna transmitter and a first input elementconfigured to receive a beam signal. A second input element in theapparatus is configured to receive an antenna signal. An estimator inthe apparatus is connected to the second receiver and is configured toestimate parameters of the received beam signal based on informationreceived in the antenna signal. The antenna signal is transmitted in acommunications system that includes multiple cells and each of the cellsincludes at least one sector. The antenna signal is transmitted over anentire sector, and the beam signal is transmitted over part of a sector.The estimator is further connected to the first input element and isconfigured to estimate the parameters based on information additionallyreceived in the beam signal.

In another aspect of the present invention, a disclosed method includesreceiving a beam signal at a receiver of a communications system, wherethis receiver includes an adaptive antenna transmitter. An antennasignal is also received at the receiver, and parameters of the receivedbeam signal are estimated based on information received in the receivedantenna signal. The antenna signal includes a common pilot channel.Also, the estimating of the parameters is further based on the receivedbeam signal and on signals received in the common pilot channel. Themethod is preferably operable in a cellular communication system thatincludes multiple cells, such that each cell includes at least onesector. The antenna signal is transmitted over an entire sector and thebeam signal is transmitted over a part of a sector.

In one aspect of the present invention, a disclosed method includesreceiving a beam signal in a receiver of a communications system. Thereceiver includes an adaptive antenna transmitter, and the communicationsystem is a cellular communication system that includes multiple cells,each having at least one sector. An antenna signal is received at thereceiver. Parameters of the received beam signal are estimated based oninformation received in the received antenna signal/Also, the antennasignal is transmitted over an entire sector and the beam signal istransmitted over a part of a sector, such that the estimating of theparameters is further based on the received beam signal. The estimatingof the parameters also includes estimating a channel of the beam signal,calculated by exploiting a statistical correlation property between thebeam signal and antenna signal.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be best understood by way of example with referenceto the following Figures in which:

FIG. 1 illustrates exemplary W-CDMA base station cells utilizingdifferent transmission schemes in each cell (three-sectorisedconfiguration); and

FIG. 2 illustrates in block diagram form elements of a receivernecessary for implementing the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, there is now described examples ofmulti-sector W-CDMA cells with respect to which the invention isillustrated. The invention is not, however, in any way limited to such aspecific example.

A plurality of mobile stations, or user equipment roam within the cell.For example, as shown in FIG. 1, mobile station 130 is connected in cell106, mobile station 132 is connected in cell 104, and mobile station 134is connected in cells 104 and 108.

The base station cell 102 is divided into N sectors, where N=3 in theexample of FIG. 1.

As exemplified by sector 106 of FIG. 1, each sector can be divided intoeither K fixed beams or steerable (user specific) beams using a basetransceiver station 112. The beams 116 represent the secondary commonpilot channel, the beam 120 the down-link dedicated physical channel,and the beam 118 the primary common pilot channel.

Sector 104 of FIG. 1 illustrates the traditional single antennatransmission scheme utilizing a base transceiver station 110. The beam120 the down-link dedicated physical channel, and the beam 118 theprimary common pilot channel.

Sector 108 of FIG. 1 illustrates user specific beam-forming using a basetransceiver station 114. The beam 120 the down-link dedicated physicalchannel, and the beam 118 the primary common pilot channel.

FIG. 1 thus illustrates the CPICHs needed in the different transmissionschemes, and the DL-CPCH of a single user.

For the purposes of describing the present invention, two of the threebase transceiver stations of the example of FIG. 1 use adaptive antennatechniques for communicating with mobile stations in the various sectorsof the cell. The two cells utilizing adaptive antenna techniques arecells 106 and 108. Adaptive antenna techniques are well-known in theart, and the present invention is not directly concerned with anyspecific implementation details of such techniques. As a skilled personwill be familiar with, when using adaptive antenna techniques the basetransceiver station 100 transmits mobile specific data to a mobilestation through a narrow beam.

The W-CDMA specification defines three different types of pilot channelsin the forward link for an adaptive antenna system. These pilot channelsare: 1. P-CPICH (Primary Common Pilot Channel); 2. S-CPICH (SecondaryCommon Pilot Channel); and 3. Dedicated pilot symbols in DPCCH(Dedicated Physical Control Channel).

The P-CPICH is broadcast over an entire sector in a multi-sectorarrangement, and there exists only one such channel for each sector. TheP-CPICH is used in the hand-over measurements and cellselection/reselection procedures. Another function of the P-CPICHchannel, when the common channels are not associated with dedicatedchannels or not involved in adaptive antenna techniques, is to aid thechannel estimation at the mobile station for the dedicated channels, andto provide a channel estimation reference for the common channels.

The S-CPICH may be transmitted over the entire cell or over only part ofthe cell. There may be zero, one or several S-CPICHs per cell or sector.One typical area of S-CPICH usage is operations with base stationshaving multiple (fixed) beams per sector. The S-CPICHs are used foridentifying different beams at the mobile station.

The dedicated pilot symbols are multiplexed into the down-link dedicatedphysical channel (DPCH). They are used in signal-to-interference ratio(SIR) estimation and are also used in the channel estimation. If themobile station or user equipment is informed that the P-CPICH is not thephase reference and there is no S-CPICH available, then the dedicatedpilot bits in the DL-DPCCH are the phase reference for the DL-DPCH. Thismay happen, for example, in the case of user-specific beam forming.

In accordance with the present invention, it is proposed to use theprimary common pilot channel P-CPICH for estimating parameters in amobile station or user equipment of the adaptive antenna system.Preferably, as described in further detail hereinbelow, the primarycommon pilot channel is used in combination with the existing channelsfor estimating parameters. Particularly advantageously, the primarycommon pilot channel is used in channel estimation in the mobilestation.

Even though user specific beam forming is applied in adaptive antennasystems, the P-CPICH must be broadcast. This means that there is astrong-powered pilot channel that is available to all mobile stations.In many cases, the SNR of the continuous and non-power-controlledP-CPICH is much better than that of time-multiplexed andpower-controlled DL-DPCCH. The relative difference of SNRs (P-CPICH vs.DL-DPCH) gets biggest when the mobile station is situated near to thebase station.

A proposed implementation of a channel estimation technique inaccordance with the present invention is provided hereinafter.

In the general case the joint channel estimation scheme is preferablydesigned to be adaptive since the channel characteristics change as afunction of time. The adaptivity of the joint channel estimator could bebased for example on the correlation measurement.

The following equations explain one possible way to implement the jointchannel estimator. The joint solution is calculated as#joint=##DL-DPCCH+(1-#)#P-CPIC (1) in which hP-CPICH and hDL-DPCCH arethe channel estimates obtained from the P-CPICH and DL-DPCCH and β isthe weight factor. The weight factor can be calculated as in which theexpectation value of the time variant correlation coefficient, /3, iscalculated as Referring to FIG. 2, there is illustrated a block diagramof the main elements of a receiver suitable for implementing the presentinvention in an exemplary W-CDMA system. Referring to FIG. 2, there isprovided three correlator banks 202,204 and 206.

There is further provided a detection and rake combiner 214, and achannel decoding block 218.

The correlator-bank 202 acts as an input means to the receiver fornormal data transmission. The correlator bank 204 acts as an input meansto the receiver for the dedicated pilot channels in the beam signal fromthe transmit antenna. The correlator bank 206 acts as an input means tothe receiver for the broadcast channels in the antenna signal from thetransmit antenna.

The three correlator banks perform the correlation for the respectivesignals, all of which are received at the receiver antenna as generallyindicated by line 200.

Each of the outputs of the correlators 202, 204, and 206 produces arespective output on lines 208,210 and 212, which form inputs to thedetection and rake combiner 214. The output of the rake combiner 214 online 216 is provided to the channel decoding block.

The channel decoded signal is then provided on line 220 for furtherprocessing.

There can be a significant correlation between the beam signal and theantenna signal in the case of user specific beam forming. Correlationbetween the P-CPICH and DL-DPCCH can be increased if the P-CPICH istransmitted via all of the antenna elements (instead of single antennaonly). The narrower the angular spread (seen from the base transceiverstation) the more correlated the antenna and beam signals are. Thecorrelation property can be seen from the fact that P-CPICH basedchannel estimation (antenna signal) performs well (depending on theangular spread) even though the DL-DPCHs are transmitted through thebeam signal. The correlation property can be exploited by using bothP-CPICH and DL-DPCCH in the channel estimation of DL-DPCH (joint channelestimation).

If the angular spread in the radio channel is small and the SNR per taplow (noise limited environment) the P-CPICH based scheme performs betterthan DL-DPCCH based channel estimation. However, at high SNR values thededicated pilot based scheme performs better than the P-CPICH basedscheme. The performance of channel estimation can thus be improved evenby selection combining (compared to P-CPICH and DL-DPCCH only basedschemes). On the other hand, if both P-CPICH and DL-DPCCH are used inthe channel estimation of DL-DPCH (joint channel estimation) all energyof the transmitted pilot signal could be utilized. Thus the jointestimation is expected to always be better than either P-CPICH norDL-DPCCH only based schemes.

The S-CPICH can also be used to aid the channel estimation at a mobilestation for the dedicated channels, for example, in the case ofmulti-beam transmission. Thus, the principle of joint parameterestimation could be applied also in the case of S-CPICH transmission(S-CPICH & P-CPICH, S-CPICH &amp; P-CPICH & DL-DPCH).

In a macro-cellular radio environment it is assumed that: the angularspread is typically relatively low; there are multiple channel taps (SNRper channel tap is low, noise limited area from the parameter estimationpoint of view); multiple channel taps (each tap is a separate cluster inthe angular domain); LOS (strong correlation, narrow angular spread);and the speed of mobile can be high.

The present invention thus provides a technique in which the correlationbetween the beam signal (DL-DPCH) and the antenna signal (P-CPICH) inthe parameter estimation (especially channel estimation) is used. Thiscorrelation is advantageously exploited in a joint channel estimationscheme, which is preferably adaptive since the correlation propertieschange as a function of time. The adaptivity of the joint channelestimator may be based, for example, on the correlation measurement.

1. A method, comprising: receiving a beam signal comprising a dedicatedchannel at a receiver in a communication system, the receiver comprisingan adaptive antenna, wherein the communication system is a cellularsystem comprising a plurality of cells, each cell comprising at leastone sector; receiving an antenna signal at the receiver, the antennasignal comprising a primary common pilot channel; and performing channelestimation on the received beam signal based on pilot signals receivedin the primary common pilot channel, wherein the antenna signal istransmitted over an entire sector and the beam signal is transmittedover part of a sector and the channel estimation is additionally basedon dedicated signals received in the dedicated channel, wherein the beamsignal comprises a secondary common pilot channel, and wherein thechannel estimation is additionally based on pilot signals received inthe secondary common pilot channel.
 2. A method according to claim 1wherein the communication system is a wideband code division multipleaccess system.
 3. A method according to claim 2, wherein the antennasignal includes a primary common pilot channel.
 4. A method according toclaim 3, wherein the estimating parameters utilises pilot signalstransmitted in the primary common pilot channel.
 5. A method accordingto claim 2, wherein the beam signal includes a secondary common pilotchannel.
 6. A method according to claim 5, wherein the estimating of theparameters utilises pilot signals transmitted in the secondary commonpilot channel.
 7. A method according to claim 2, wherein the beam signalincludes a dedicated physical channel.
 8. A method according to claim 7,wherein the estimating of the parameters utilises signals transmitted inthe dedicated physical channel.
 9. A method according to claim 1 inwhich the estimating of the parameters includes channel estimation. 10.A method according to claim 9 wherein the channel estimate of the beamsignal is calculated by exploiting the statistical properties betweenthe beam signal and antenna signal.
 11. A method according to claim 9wherein the channel estimate of the beam signal is calculated byexploiting a priori knowledge of the antenna signal.
 12. A methodaccording to claim 11 wherein the channel estimate of the beam signal iscalculated by exploiting further a priori knowledge of the beam signal.13. A method according to claim 1, wherein the performing the channelestimation comprises estimating a channel of the beam signal, calculatedby exploiting a statistical correlation property between the beam signaland antenna signal.
 14. An apparatus, comprising: an adaptive antenna; afirst input element configured to receive a beam signal comprising adedicated channel; a second input element configured to receive anantenna signal comprising a primary common pilot channel; and anestimator, connected to the second input element, configured to performchannel estimation on the received beam signal based on pilot signalsreceived in the primary common pilot channel, wherein the antenna signalis transmitted in a communications system, the communications systemcomprising a plurality of cells and each cell comprising at least onesector and the antenna signal is transmitted over an entire sector andthe beam signal is transmitted over part of a sector, wherein the beamsignal comprises a secondary common pilot channel, and wherein theestimator is further connected to the first input element, andconfigured to perform the channel estimation based on dedicated signalsreceived in the dedicated channel and pilot signals received in thesecondary common pilot channel.
 15. An apparatus according to claim 14,wherein the apparatus is configured to be used in a wideband codedivision multiple access system.
 16. An apparatus according to claim 15,wherein the antenna signal includes a primary common pilot channel. 17.An apparatus according to claim 15, wherein the beam signal includes asecondary common pilot channel.
 18. An apparatus according to claim 14,wherein the apparatus is configured to be used in a mobile station in awideband code division multiple access system.
 19. An apparatusaccording to claim 15, wherein the beam signal includes a dedicatedphysical channel.
 20. An apparatus according to claim 19 wherein theestimation of parameters utilises signals transmitted in the dedicatedchannel.
 21. An apparatus according to claim 14, wherein the estimatingparameters utilises pilot signals transmitted in the primary and/orsecondary common pilot channel.
 22. An apparatus according to claim 14in which the estimation of parameters includes channel estimation.